1. Field of the Invention
This invention relates to scanning probe microscopy, and in particular to the preparation of force-sensing cantilevers for use in scanning probe microscopy and atomic force microscopy.
2. The Prior Art
In a conventional atomic force microscope or AFM, the deflection of a flexible cantilever is used to monitor the interaction between a probe tip disposed on an end of the cantilever and a surface under study. As the probe tip is brought close to the surface, it deflects in response to interactions with the surface under study. These deflections are used to control the distance of the tip from the surface and to measure details of the surface as explained, for example, in "Atomic Force Microscope", G. Binnig, C. F. Quate and C. Gerber, Physical Review Letters, Vol. 56, No. 9, pp. 930-933 (1986).
It is often desirable to operate an atomic force microscope in an oscillating mode--this is referred to in the art as the "AC mode". In the AC mode, the cantilever is vibrated at a high frequency (typically somewhere in the range of about 1 kHz to about 100 kHz), and the change in amplitude of the physical oscillations of the cantilever or phase of those oscillations with respect to the driving signal as the cantilever approaches a surface under study is used to control the microscope as explained, for example, in "Frequency Modulation Detection Using High-Q Cantilevers for Enhanced Force Microscopy Sensitivity", T. R. Albrecht, P. Gruitter, D. Horne and D. Rugar, Journal of Applied Physics, Vol. 69, pp. 668-673 (1991). One reason for using the AC mode to vibrate the cantilever is that by doing this, when oscillated at high amplitude, the probe tip is less likely to stick to the surface under study if it should come into contact with it. This is explained, for example, in "Fractured Polymer/Silica Fibre Surface Studied by Tapping Mode Atomic Force Microscopy", Q. Zhong, D. Inniss, K. Kjoller and V. B. Elings, Surface Science Letters, Vol. 290, pp. L688-L692 (1993). However, the AC mode of operation is also intrinsically more sensitive than other known methods of operating an atomic force microscope. AC detection shifts the signal to be detected to sidebands on a carrier signal, avoiding the low frequency noise that DC signals suffer from. In addition, the mechanical "Q" of a cantilever resonance can be used to enhance the overall signal to noise ratio of a microscope operated in this fashion as explained in "Frequency Modulation Detection Using High-Q Cantilevers for Enhanced Force Microscopy Sensitivity", T. R. Albrecht, P. Grutter, D. Horne and D. Rugar, Journal of Applied Physics, Vol. 69, pp. 668-673 (1991).
One version of an AC mode operated atomic force microscope, where contact between the probe tip and the surface under study is actually sought out, is described in U.S. Pat. No. 5,412,980 to V. Elings and J. Gurley entitled "Tapping Atomic Force Microscope" and in U.S. Pat. No. 5,519,212 to V. Elings and J. Gurley entitled "Tapping Force Microscope with Phase or Frequency Detection". In this version, where the oscillatory mode of operation is referred to as the "tapping mode" for the fact that it actually repeatedly taps at the surface under study, the oscillation is used mainly as a means of avoiding the effects of adhesion between the probe tip and the surface being "tapped". It has also been discovered that such adhesion may easily be overcome by chemical means. For example, the microscope may be operated in a fluid which minimizes adhesion. Alternatively (or additionally) a tip material may be chosen so as to minimize its adhesion to the surface under study. Where such techniques are used to reduce adhesive forces, there is no longer any reason to operate the atomic force microscope at a large oscillation amplitudes.
The usual method of exciting motion in the cantilever of an atomic force microscope is to drive it with an acoustic excitation. This method works well in air or gas and has been made to work with the tip submerged in water. See, for example, "Tapping Mode Atomic Force Microscopy in Liquids", P. K. Hansma, J. P. Cleveland, M. Radmacher, D. A. Walters, P. E. Hilner, M. Bezanilla, M. Fritz, D. Vie, H. G. Hansma, C. B. Prater, J. Massie, L. Fukunaga, J. Gurley and V. B. Elings, Applied Physics Letters, Vol. 64, pp. 1738-1740 (1994) and "Tapping Mode Atomic Force Microscopy in Liquid", C. A. J. Putman, K. 0. V. d. Werf, B. G. deGrooth, N. F. V. Hulst and J. Greve, Applied Physics Letters, Vol. 64, pp. 2454-2456 (1994). However, in a fluid, the motions of the cantilever become viscously damped, so that substantial acoustic amplitude is required to drive and sustain the desired oscillatory motion of the cantilever. Furthermore, the fluid acts as a coupling medium between the source of acoustic excitation and parts of the microscope other than the cantilever. The result of this undesired coupling is that parts of the microscope other than the cantilever get excited into motion by the acoustic signal used to vibrate the cantilever. If such motion leads to a signal in the detector, a background signal is generated which is spurious and not sensitive to the interaction between the tip and surface as explained in "Atomic Force Microscope with Magnetic Force Modulation", E. L. Florin, M. Radmacher, B. Fleck and H. E. Gaub, Review of Scientific Instruments, Vol. 65, pp. 639-643 (1993). S. M. Lindsay and co-workers have described a scheme for exciting the cantilever directly while avoiding unwanted mechanical coupling of an oscillatory signal into the larger structure of the atomic force microscope. This is described at, for example, "Scanning Tunneling Microscopy and Atomic Force Microscopy Studies of Biomaterials at a Liquid--Solid Interface", S. M. Lindsay, Y. L. Lyubchenko, N. J. Tao, Y. Q. Li, P. I. Oden, J. A. DeRode and J. Pan, Journal of Vacuum Science Technology A, Vol. 11, pp. 808-815 (July/August 1993). In this approach, a magnetic particle or film is attached to the cantilever and a solenoid near the cantilever is used to generate a controlled magnetic force on the cantilever. This arrangement gives extreme sensitivity to surface forces as discussed in "Atomic Force Microscopy of Local Compliance at Solid--Liquid Interfaces", S. J. O'Shea, M. E. Welland and J. B. Pethica, Chemical Physical Letters, Vol. 223, pp. 336-340 (1994), presumably because of a lack of the background spurious signals that would normally be present in an acoustically-excited atomic force microscope. It is the basis of a novel form of AC-AFM in which the cantilever is excited by magnetic means (see, for example, U.S. Pat. No. 5,515,719 entitled "Controlled Force Microscope for Operation in Liquids" to S. M. Lindsay and U.S. Pat. No. 5,513,518 entitled "Magnetic Modulation of Force Sensor for AC Detection in an Atomic Force Microscope" to S. M. Lindsay).
Magnetic cantilevers are required in order to operate such a microscope. Three approaches have been used. First, Lindsay et al. described a method for fixing a magnetic particle onto the cantilever ("Scanning Tunneling Microscopy and Atomic Force Microscopy Studies of Biomaterials at a Liquid--Solid Interface", S. M. Lindsay, Y. L. Lyubchenko, N. J. Tao, Y. Q. Li, P. I. Oden, J. A. DeRode and J. Pan, Journal of Vacuum Science Technology, Vol. 11, pp. 808-815 (1993)). This method, however, does not readily permit fabrication of suitable cantilevers in quantity. Second, O'Shea et al. ("Atomic Force Microscopy of Local Compliance at Solid--Liquid Interfaces", S. J. O'Shea, M. E. Welland and J. B. Pethica, Chemical Physical Letters, Vol. 223, pp. 336-340 (1994)) describe a method for evaporating a magnet coating onto the cantilever. In order to avoid bending the cantilever due to the interfacial stress introduced by the evaporated film, they place a mask over most of the cantilever so that the magnetic film is deposited only onto the tip of the force-sensing cantilever. This approach requires precision alignment of a mechanical mask and it is not conducive to simple fabrication of suitable coated cantilevers. Third, in an earlier invention by one of the present inventors (S. M. Lindsay, U.S. patent application Ser. No. 08/553,111 entitled "Formation of a Magnetic Film on an Atomic Force Microscope Cantilever") methods for coating cantilevers with magnetic films and for calibrating the properties of the films were described. However, while suited for its intended purposes, the yield of cantilevers produced by this process was not as high as desired as some of the cantilevers coated on one side only were manufactured with unacceptable bends or deformations.
Magnetic cantilevers have also been manufactured for the purpose of magnetic force microscopy, an imaging process which is used to sense local magnetization in materials such as magnetic recording media and recording heads. Grutter et al. (P. Grutter, D. Rugar, H. J. Mamin, G. Castillo, S. E. Lambert, C. J. Lin, R. M. Valletta, O. Wolter, T. Bayer and J. Greschner, "Batch Fabricated Sensors for Magnetic Force Microscopy", Applied Physics Letters, Vol. 57, No. 17, pp. 1820-1822, 22 Oct. 1990) describe a method for coating silicon cantilever probe tips with thin (15 nm) films of Cobalt, Cobalt alloys (Co/Pt/Cr) and Permalloy (Ni/Fe). These devices are typically 500 .mu.m long, 11.5 .mu.m wide, 5-7 .mu.m thick with a spring constant on the order of 1 N/m and resonant frequencies of about 35 kHz. Babcock et al. (K. Babcock, V. Elings, M. Dugas and S. Loper, "Optimization of Thin-Film Tips for Magnetic Force Microscopy", I.E.E.E. Transactions on Magnetics, Vol. 30, No. 6, pp. 4503-4505, November, 1994) describe sputtering thin films (500 .ANG.) of Cobalt alloy (Co/Cr) onto the probe tips of silicon cantilevers having lengths of 225 .mu.m widths of 27 .mu.m, thickness of 3 .mu.m, resonant frequencies of 92.4 kHz and spring constants of about 5 N/m . In each case, only the probe tip side of the cantilever was coated with the thin film. While suited for the task of Magnetic Force Microscopy, silicon cantilevers are expensive to fabricate and the yield is substantially lower than the yield for silicon nitride cantilevers. Silicon cantilevers are also generally much stiffer than silicon nitride cantilevers (with corresponding spring constants of about 50 N/m for silicon and less than 1 N/m for silicon nitride) and these "softer" cantilevers are desirable for imaging soft samples such as biological samples and the like. Silicon nitride cantilevers, however, suffer from the drawback that, because they are soft, they are easily bent upon being coated with a sputtered film.
Accordingly, it would be desirable to provide a magnetic cantilever and method for fabricating the same which results in higher yields and more ready manufacturability than heretofore available while inducing minimum bending and without requiring precision masks to cover parts of the cantilevers during deposition of a magnetic film.